Relative to the mouse system, the chicken system has significant advantages, and substantial disadvantages. On the plus side, the developing chicken embryo is relatively large, and the fact that it develops outside the mother makes it much easier to manipulate. On the minus side, chicken genetics are considerably less powerful than mouse genetics. We have tried to make it possible to use the RCAS vectors both in the avian system and in the mouse system, so that the experimentalist can have the best of both worlds.
The experiments done with chickens can be (somewhat arbitrarily) divided into those done with embryos, and those done post-hatch. For the most part, experiments done post-hatch are intended to look at biological questions or to investigate the effects of particular oncogenes. It is possible to generate transgenic buds by using RCAS vectors to infect cells that contribute to the germline. However, the most common use of the RCAS system in chickens is to explore the effects of the expression of individual genes on embryonic development. In the simplest form of these experiments, the virus is introduced into a particular tissue or organ and allowed to spread. If the size of the inoculum and the duration of the experiment are appropriately chosen, the effects of the RCAS vector — and, more particularly, the gene it carries — are confined to the desired tissue or organ. If this is not sufficient, the spread of the virus can be confined using surgically constructed chimeric embryos. Not all strains of chickens express functional receptors for all the ASLVs. If a chimeric embryo is constructed from tissues from a permissive and a nonpermissive embryo, the virus will be confined to those tissues derived from the permissive embryo (Fekete and Cepko, 1993). It is also possible to infect chicken embryos with replication-defective versions of RCAS. If the stocks of the replication-defective vector are carefully prepared (i.e., free of replication-competent helper virus), the vector will not spread in the embryo. This system can also be used to follow cell fate/lineage in a developing embryo.
In thinking about experiments infecting embryos with RCAS vectors, it is worth considering the use of infected cells instead of virus. Infected cells are an excellent source of virus and can, at least in some cases, be delivered with more precision and in a smaller volume than a viral stock.